function fetchData() {
    // 性能 
    let res1 = yield ajax(url,() => {})
    let res2 = yield ajax(url1,() => {})
    let res3 = yield ajax(url2, () => {})
}
let it = fetchData() // generator
let result1 = it.next()
let result2 = it.next()
let result3 = it.next()

// 缺点?